1. Field of the Invention
The present invention relates to a quinoxaline derivative, and a light-emitting element and light-emitting device using the quinoxaline derivative.
2. Description of Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. As a basic structure of these light-emitting elements, a structure where a substance having a light-emitting property is interposed between a pair of electrodes is used. By application of a voltage to this element, light emission from a substance having a light-emitting property can be obtained.
Since such a light-emitting element is a self-luminous element, there are advantages that visibility of a pixel is better than visibility of a liquid crystal display, that a backlight is not necessary, and the like. Accordingly, such a light-emitting element is considered suitable as a flat panel display element. In addition, such a light-emitting element can be manufactured to be thin and light, which is a great advantage. Moreover, the light-emitting element has a feature that response speed is extremely fast.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained by formation of a large-area element. This characteristic is difficult to be obtained by a point light source typified by an incandescent lamp or an LED, or a line light source typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a plane light source that can be applied to lighting or the like.
The light-emitting elements using electroluminescence are classified roughly in accordance with whether they use an organic compound or an inorganic compound as a substance having a light-emitting property.
In a case where a substance having a light-emitting property is an organic compound, by application of a voltage to the light-emitting element electrons and holes are injected from the pair of electrodes into the layer including an organic compound having a light-emitting property to cause current flow. Then, by recombination of these carriers (electrons and holes), the organic compound having a light-emitting property gets in an excited state, and light is emitted when the excited state returns to a ground state. Because of such a mechanism, this kind of light-emitting element is referred to as a light-emitting element of a current excitation type.
It is to be noted that an excited state formed by an organic compound can be a singlet-excited state or a triplet-excited state. Light emission from the singlet-excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In order to overcome many problems derived from materials of such a light-emitting element and to improve its element characteristics, improvement in an element structure, material development, and so on are carried out.
For example, when the light-emitting element is used for a lighting system, the color rendering property of light emission of the light-emitting element becomes a concern. When a white light-emitting element is manufactured using a plurality of light-emitting materials, the color rendering property becomes low if the emission spectrum of each light-emitting material is sharp. On the other hand, if the emission spectrum is broad, the color rendering property becomes high because light emission occurs in the entire visible light region; accordingly, light emission that is close to that of natural light can be obtained.
However, with the light-emitting element using a plurality of light-emitting materials, it is difficult to adjust the balance of light emitted from the light-emitting materials. In addition, since the light-emitting materials differ in how easily they become degraded, even if the light-emitting element provides target white light emission when it is first manufactured, the luminance balance of the light-emitting materials changes due to the degradation, and there is a problem that the light emission color of the light-emitting element changes.
In Patent Document 1: Japanese Published Patent Application No. 2003-203780, a white light-emitting compound including a quinacridone skeleton and a carbazole skeleton is disclosed. However, there are still not very many compounds capable of white light emission, and the development of a novel white light-emitting compound is demanded.